The present invention relates to a conjugate of methotrexate to an antibody, in which the methotrexate is first loaded onto an aminodextran and this intermediate is in turn site-specifically conjugated to an antitumor antibody. The resultant conjugate targets the methotrexate cytotoxicity to a tumor cell for a therapeutic result.
Conjugation of cytotoxic drugs to antibodies to achieve a targeted therapeutic result is known. In particular, it is known that methotrexate (MTX) can be conjugated to antibodies and some selective cytotoxicity has been observed. It is desirable to enhance the selectivity and cytotoxicity of such conjugates by increasing the antibody loading of the cytotoxic drug. However, multiple conjugation of individual drug molecules to an antibody eventually reduces its immunoreactivity, the effect being observed when more than about 10 drug molecules are loaded.
It has been suggested that the drug be conjugated to an intermediate polymeric carrier, which in turn would be conjugated to antibody. This has the advantage that larger numbers of drug molecules can be attached to the antibody at fewer sites on the antibody itself, so that immunoreactivity is not as seriously compromised.
One approach has been to attach MTX to bovine serum albumin (BSA) and then randomly link the intermediate to antibody, as reported by Garrett et al., Int. J. Cancer, 31: 661-670, 1983. These authors were able to attach about 37 MTX molecules to BSA (average molecular weight of 70,000) but the resultant antibody conjugate had an immunoreactivity of only about 28% of that of the intact antibody.
Use of polylysine as a polymer carrier was reported by Ryser et al., Proc. Natl. Acad. Sci. USA, 75: 3867-3870, 1978. These authors found that only about 13 MTX per carrier could be loaded and immunoreactivity was poor. In addition, the high amine content of the polymer, largely in the form of charged ammonium groups, caused the conjugate to stick to normal cells and vitiated the selectivity of the cytotoxic effect.
A need therefore continues to exist for an antibody conjugate of MTX that combines high loading with minimal decrease in immunoreactivity for selective targeting of MTX to tumor cells.